As a wood and metal refinisher for over 20 yrs. polyester is not the ideal finish for a chassis.Other than having it coated in a place (like AZ) where the humidity is low 5-15% you always trap moisture under the coating. Polyester ia also very sensitive to unclean sufaces. Having stripped many metal surfaces coated with polyester, very few items are ever rust free when the coating has been removed. Using an epoxy primer with either a single or 2 stage topcoat protects the surface from the elements, is easy to touch up, easier to remove when needing to reweld a spot and will not hide rust and any structural problems with a chassis. Plus you can do it in your shop and save $.